Introduced nearly three decades ago, DTMF (dual-tone multi-frequency) signaling has been widely used as a signaling method in telephone central offices, private branch exchanges, and private automatic branch exchange equipment. DTMF is also known under the mark TOUCH-TONE, a registered trademark of American Telephone and Telegraph Corporation. DTMF is also used for communications and in many other applications such as radio telephones, electronic tellers, banking/financial transactions via remote data entry, remote security and alarm activation systems, remote control systems, remote data-loggers, telemetry, toll-call restrictors, touch-tone to dial-pulse conversion and dial-pulse to touch-tone areas.
There are basically two types of DTMF receivers: analog and digital. Analog DTMF receivers typically require more support chips such as a CODEC and an eight-bit microcontroller. Digital DTMF receivers do not require as many support chips because the DTMF signal is already in digital form as PCM (pulse code modulation) data. Digital DTMF receivers can be incorporated into such digital signal processor chips as members of the Texas Instruments TMS320 family, which can handle PCM inputs without the use of a CODEC. The TMS320 can also act as a powerful controller.
Most existing DTMF decoders employ digital filtering techniques. Discrete Fourier transform, fast Fourier transform and other spectral and correlation techniques are both more time consuming and less desirable since standard DTMF tones are harmonically unrelated to each other.
Unfortunately, most digital filtering techniques require an excessively long cycle time for front-end filtering, leaving very little real time for post-detection digit validation and for other system software overhead. Also, filtering alone is not enough to distinguish between speech and valid DTMF digits in order to meet the AT&T standard specifications for digit simulations or talk-off performance. This is largely because speech signals have spectra spanning across the entire 4 KHz range. PCM data sampled at 8 KHz can only have a maximum bandwidth of 4 KHz in order to satisfy the sampling theorem. As a result, most commercial DTMF decoder chips do not perform well in digit simulations.
Therefore, a need has arisen for a digital DTMF receiver that can adequately discriminate between valid dual-tone pairs and other signal sources such as speech or noise, and further which has a sufficiently short cycle time to be useful for commercial applications.